Airway response to deep inspiration: role of inflation pressure

Deep inspirations (DIs) have been shown to have both bronchoprotective and bronchodilator effects in healthy subjects; however, the bronchodilator eff ects of a DI appear to be impaired in asthmatic compared with healthy subje cts. Because the ability to generate high transpulmonary pressures at total lung capacity depends on both the lung properties and voluntary effort, we wondered how the response of airways to DI might be altered if the maneuve r were done with less than maximal inflation. The present work was undertak en to examine the effects of varying the magnitude of lung inflation during the DI maneuver on subsequent airway caliber. In five anesthetized and ven tilated dogs during methacholine infusion, changes in airway size after DIs of increasing magnitude were measured over the subsequent 5-min period usi ng high-resolution computed tomography. Results show that the magnitude of lung inflation is extremely important, leading to a qualitative change in t he airway response. A large DI (45 cmH(2)O airway pressure) caused subseque nt airway dilation, whereas smaller DIs (less than or equal to 35 cmH(2)O) caused bronchoconstriction. The precise mechanism underlying these observat ions is uncertain, but it seems to be related to an interaction between int rinsic properties of the contracted airway smooth muscle and the response t o mild stretch.